<volist name="field.opt" id="option">
    <php>
        $checked = $field['value']==$key ? 'checked' : '';
        $inputId = "id_$field[name]_$key";
    </php>
    <input id="{$inputId}" name="{$field.name}" value="{$key}" type="radio" {$checked}/>
    <label for="{$inputId}">{$option|htmlspecialchars}</label>
</volist>